Ras is a membrane-associated guanine-nucleotide-binding protein that plays a key role in many cellular processes, including cell proliferation, apoptosis, differentiation, senescence, and survival. Ras is an ON-OFF switch for such cellular processes. While it is normally at rest (OFF) and bound to the guanosine diphosphate (GDP) nucleotide, Ras can be activated (ON) when bound to the guanosine triphosphate (GTP) nucleotide by extracellular signaling molecules that act on a variety of targets.
Ras proteins play a key role in tyrosine kinase growth-factor receptor signaling (Egan, S. E. and Weinberg, R. A. Nature 365, 781-782 (1993); McCormick, F., Nature, 363, 15-16 (1993)). When activated in the GTP-bound form, Ras proteins propagate the growth factors' signal to the MAP kinase cascade. Ras proteins are associated with the plasma membrane where activation of the Raf kinase occurs through a direct Ras/Raf interaction (Zheng, X. F. et al., Nature, 364, 308-313 (1993); Warne, P. H., Nature, 364, 352-353 (1993)).
Termination of growth factor signaling involves hydrolysis of the active, GTP-bound form of Ras to the inactive, GDP-bound form of Ras. However, mutated or oncogenic Ras proteins do not hydrolyze GTP and are therefore in a permanently active (ON) state. The inability to hydrolyze GTP may contribute to various uncontrolled cellular functions.
Activated Ras can initiate and drive malignant cell growth of tumor cells, including tumor cells that express activated Ras proteins. Mutated Ras proteins are found at high frequencies in human cancers (Bos, J. L. Cancer Res., 49, 682-4689 (1989); Barbacid, M., An. Rev. Biochem, 56, 779-829 (1987)). In some types of tumors, such as colon and pancreatic carcinomas, the incidence of activated Ras is higher than 50%. In addition to tumors that result from the unbridled actions of mutated or oncogenic Ras, there are also tumors that are caused by constitutively active growth factor receptors (e.g., the Epidermal Growth Factor receptor, the Fibroblast Growth Factor receptor, and the Platelet-Derived Growth Factor receptor) that hold Ras in the active (ON) position. Therefore, pharmacological methods to affect Ras activity may be of use for the treatment of certain types of human cancers.
In addition to malignant cancer, activated Ras can initiate and drive non-malignant (i.e., benign) cellular proliferation. One example of Ras-induced, non-malignant cellular proliferation is psoriatic lesions. An increased level of activated Ras has been found in psoriatic lesion of patients. (Lin F., Baldassare, J. J., Voorhees, J. J., Fisher, G. J. Increased activation of Ras in psoriatic lesions. Skin Pharmacol Appl Skin Physiol 1999 January-April; 12(1-2):90-7). In addition, receptor signaling via the Ras/MAPK cascade has been identified as playing a key role in psoriatic lesions (Mark, E. B., Jonsson, M., Asp, J., Wennberg, A. M., Molne, L., Lindahl, A. Expression of genes involved in the regulation of p16 in psoriatic involved skin. Arch Dermatol Res 2006 April; 297(10):459-67. Epub 2006 Mar. 22).
Other examples of Ras-induced, non-malignant cellular proliferation are found in a variety of inherited diseases (e.g., neurofibromatosis type 1 (NF-1) and polycystic kidney disease (PKD)) and diverse sporadic problems such as pulmonary, hepatic, renal, and cardiac fibrosis. For example, neurofibromin is a Ras GTPase activating protein (Ras-GAP) that helps turn off activated Ras and is therefore a tumor suppressor. Genetic mutations leading to the decreases in neurofibromin lead to NF-1, a condition where tumors grow on the nerve tissue, because the activated, GTP-bound Ras cannot revert to inactivated, GDP-bound Ras. These tumors may be non-malignant (i.e., benign), but depending on their location, they may cause serious damage to surrounding tissues. In addition, these growths may transform into malignant nerve sheath tumors. Leukemia is also more common in NF-1 patients than it is in the general population. As another example, autosomal dominant PKD is a proliferation of renal epithelial cells and subsequent cyst formation. Inhibition of Ras stops the aberrant growth of these cells. (Parker, E., Newby, L. J., Shaprpe, C. C., Rossetti, S., Streets, A. J., Harris, P. C., O'Hare, M. J., Ong, A. C. Hyperproliferation of PKD-1 cystic cells is induced by insulin-like growth factor-1 activation of the Ras/Raf signaling system. Kidney Int 2007 July: 72(2): 157-65. Epub 2007 Mar. 28).
Still another example of Ras-induced, non-malignant cellular proliferation is found in the pathological state of postangioplasty restenosis following the placement of stents in arteries, which results from the proliferation of vascular endothelial cells. Such cell proliferation may be initiated by tissue injury or damage (e.g., damage caused by insertion of the stent) or local vascular inflammation, for example.
In addition to driving cellular proliferation and tumorigenesis, Ras activation mediates a number of immune phenomena and abnormalities in immune function, such as those seen in autoimmune diseases. These autoimmune diseases can be Ras dependent. Autoimmune diseases are characterized by self-inflicted tissue damage. Any organ may be affected by such processes through precipitation of immune complexes, cellular immunity, or inappropriate generation or action of proinflammatory immuno-hormones such as cytokines. Autoimmune diseases are a significant public health problem because of the numbers of patients that they affect and the morbidity and mortality that they cause. Common chronic systemic diseases in this group include type 1 diabetes mellitus, Hashimoto's thyroiditis, rheumatoid arthritis, systemic lupus erythematosus (SLE), primary antiphospholipid syndrome (APS), and a variety of diseases that affect the central and peripheral nervous systems, including myasthenia gravis, Lambert Eaton myasthenic syndrome, Guillain-Barre syndrome, polymyositis, and multiple sclerosis. In addition, there are neurological complications of the systemic autoimmune diseases. The sensory neuropathy associated with type 1 diabetes is an example. Factors contributing to autoimmune diseases include genetic predisposition and environmental agents (e.g., certain infections and pharmaceutical products). The rejection of cells and tissues following organ transplantation is another immune system mediated phenomenon in which Ras has been implicated (Trujillo, J. I., Expert Ipin Ther Pat. 21, 1045-1069 (2011)), as is chronic graft versus host disease (Svegliati S, Olivieri A, Campelli N, Luchetti M, Poloni A, Trappolini S, Moroncini G, Bacigalupo A, Leoni P, Avvedimento EV, and Gabrielli A. Blood 110, 237-241 (2007).
Just as abnormalities in Ras signaling drive pathological immune responses, activated Ras can contribute to the dysregulation of other body systems as well, such as the endocrine system and the vascular system. An example is the faulty control of insulin sensitivity in peripheral tissues and ultimately the failure of the pancreas in type 2 diabetes. Ras antagonists can reverse insulin resistance in animal models of this disease. (Mor, A., Aizman, E., George, J., Kloog, Y. Ras Inhibition induces insulin sensitivity and glucose uptake. PLoS One 2011 6(6):e21712. Epub 2011 Jun. 29). Another example is vascular inflammation which is driven by proinflammatory adipokines in obese animals and humans and which contributes to the pathology of diabetes and atherosclerosis. (George, J., Afek, A., Keren, P., Herz, I., Goldberg, I., Haklai, R., Kloog, Y., Keren, G. Functional inhibition of Ras by a Ras antagonist attenuates atherosclerosis in apolipoprotein E knockout mice. Circulation 2002, 105(20): 2416-2422).